Department of Meteorology, University of Reading, Reading, UK.
Institute of Physics, University of Tartu, Tartu, Estonia.
Nature. 2019 Aug;572(7767):51-55. doi: 10.1038/s41586-019-1423-9. Epub 2019 Jul 31.
The cooling of the Earth's climate through the effects of anthropogenic aerosols on clouds offsets an unknown fraction of greenhouse gas warming. An increase in the amount of water inside liquid-phase clouds induced by aerosols, through the suppression of rain formation, has been postulated to lead to substantial cooling, which would imply that the Earth's surface temperature is highly sensitive to anthropogenic forcing. Here we provide direct observational evidence that, instead of a strong increase, aerosols cause a relatively weak average decrease in the amount of water in liquid-phase clouds compared with unpolluted clouds. Measurements of polluted clouds downwind of various anthropogenic sources-such as oil refineries, smelters, coal-fired power plants, cities, wildfires and ships-reveal that aerosol-induced cloud-water increases, caused by suppressed rain formation, and decreases, caused by enhanced evaporation of cloud water, partially cancel each other out. We estimate that the observed decrease in cloud water offsets 23% of the global climate-cooling effect caused by aerosol-induced increases in the concentration of cloud droplets. These findings invalidate the hypothesis that increases in cloud water cause a substantial climate cooling effect and translate into reduced uncertainty in projections of future climate.
地球气候因人为气溶胶对云的影响而冷却，抵消了温室气体变暖的未知部分。气溶胶通过抑制雨水形成，增加液相比云中的水量，据推测会导致大量冷却，这意味着地球表面温度对人为强迫非常敏感。在这里，我们提供了直接的观测证据，表明与未受污染的云相比，气溶胶导致液相比云中的水量平均减少，而不是强烈增加。对各种人为源下风处的污染云（如炼油厂、冶炼厂、燃煤电厂、城市、野火和船只）的测量表明，气溶胶引起的云水量增加（由于抑制雨水形成）和云水量减少（由于云水量蒸发增强）部分相互抵消。我们估计，观测到的云水量减少抵消了气溶胶引起的云滴浓度增加导致的全球气候冷却效应的 23%。这些发现否定了云水量增加会导致大量气候冷却效应的假设，并降低了未来气候预测的不确定性。
